Eighty patients were recruited into a double-blind, randomized trial to find the optimal dose of droperidol for addition to the patient-controlled analgesia (PCA) morphine infusate for female patients undergoing gynaecological surgery. A standardized anaesthetic technique was employed. Post-operative analgesia was provided by PCA morphine. Patients were allocated at random into one of four treatment groups receiving with each PCA morphine bolus: (1) droperidol 0.05 mg; (2) droperidol 0.10 mg; (3) droperidol 0.15 mg; and (4) droperidol 0.20 mg, respectively. The incidence of post-operative nausea and vomiting (PONV), requests for rescue anti-emetic medication, and incidence of side effects were recorded. The number of symptom-free patients in each group increased as the droperidol dose increased, but although there was a significant inverse association between the total dose of droperidol received and the severity of PONV (P<0.05), there were no significant differences between individual groups. In each group, patients were significantly less sedated at 24 h compared with 12 h (P<0.01). However, after 24 h, patients in group 4 were significantly more sedated than patients in groups 1 and 2 (P<0.05). There were no significant differences between the groups in terms of the incidence of anxiety or other side effects attributable to droperidol. The present authors suggest that, although the results demonstrate few statistically significant differences between the four groups, a PCA bolus dose of droperidol of 0.10 mg mL−1 appears to provide the optimal balance between anti-emetic efficacy and an acceptable incidence of side effects.